This invention relates to an integrated circuit with protection against reversed power supply and more particularly to such a circuit wherein the hot (Vcc) power supply terminal pad is connected through a fusible link and a normally reverse biased PN junction from an epitaxial pocket to the IC substrate, and wherein each ohmic connection between each of the other epitaxial pockets and the hot terminal pad are effected through a polysilicon resistor.
Integrated circuits (IC's) having conventional PN junction isolation of individual components are typically designed to perform digital or analogue functions. They are often powered from a battery, e.g. when used in automobiles, portable equipment or in toys. In these battery powered applications, the danger exists that when replacing a used battery with a fresh one, the new battery may be connected backward from normal.
Most semiconductor integrated circuits are so constructed that a polarity reversal of the d.c. power source will cause permanent and catastrophic damage to the circuit components. It is desirable that low cost protective means against an inadvertently reversed battery, be provided within the integrated circuit itself. A known means for accomplishing this, comprises forming by standard IC methods a normal p-n junction diode in the circuit substrate and connecting this diode in series relationship with the d.c. power supply (battery) and d.c. power buss terminals of the primary portion of the IC. The diode is polarized so that it is "forward biased" when the battery is properly connected and vice versa. Unfortunately, the simplest means for fabricating such a protective diode often results in the formation of a parasitic transistor that degrades the performance of the primary circuit portion. Also, the forward drop of the protective diode, e.g. 0.6 volts, is often intolerable.
An alternative solution is described in the patent to R. Genesi, U.S. Pat. No. 3,940,785 issued Feb. 24, 1976 that is assigned to the same assignee as is the present invention. The epitaxial pockets, containing a diffused resistor or a transistor, which pockets would normally connect directly to the hot (Vcc) terminal pad, are instead connected via a Schottky diode for reversed battery protection. However, even the low forward drop of Schottky diodes may be objectionable for operation at low power supply voltages, and on the other hand the reverse breakdown voltage of the usual integrated Schottky diode is about 20 volts limiting the supply voltage that may be used to less than 20 volts.
It is an object of the present invention to provide an integrated circuit with reverse supply protection that overcomes the above above noted limitations of the prior art.